Spread-spectrum communications systems spread the intended information signal over a wide band of frequencies and/or time segments in order to hide the signal from unintended recipients. Typically, a pseudo-random code known only by the sender and the intended recipient is used to spread the information signal. If the bandwidth over which the signal is spread is large enough, unintended recipients that do not know the pseudo-random code may not be able to distinguish the spread signal from a noise signal.
The traditional means of producing a spread-spectrum signal is to begin with a continuous wave (CW) carrier. This carrier is then modulated by a high data-rate pseudo-random code and a lower data-rate information signal. Various modulation schemes are used (i.e., phase-shift keying). Due to the high data-rate of the pseudo-random code, the signal acquires a large bandwidth. The rate of the code determines the bandwidth of the transmitted signal. A high code rate leads to a high bandwidth transmission.
In order to recover the information signal, the intended receiver must cross-correlate the spread-spectrum signal received using the same pseudo-random code in the same sequence used in the transmitter to spread the transmission. If the energy per unit bandwidth of the transmitted signal is below the average noise level seen by the receiver, an uncorrelated receiver (i.e., a receiver that does not use the same pseudo-random code and/or sequence) will not detect the presence of a spread-spectrum signal. However, a receiver that cross-correlates the transmission will exhibit an improvement in signal to noise ratio (SNR) over the uncorrelated receiver. The improvement in SNR is limited by the process gain, Gprocess, which is given by: EQU Gprocess=BW/Rinfo,
where BW is the bandwidth over which the transmitted signal was spread and Rinfo is the information data rate (i.e., the data rate of the signal being spread). Hence, even if an uncorrelated receiver is not able to recover the transmitted signal, a correlated receiver may well be able to recover the signal by utilizing the process gain. The correlated receiver, however, requires the use of the pseudo-random code used by the transmitter. If the secrecy of the code is maintained between the sender and intended recipient then unintended recipients of the spread-spectrum transmission may not be able to recover the underlying signal. Typically, electronic parts are used to realize the above-identified approach. The primary problem with using such devices is the relatively slow speed of electronic devices. The speed limitation appears in both the transmitter and the receiver.
The bandwidth of the transmitted signal is determined by the data rate of the pseudo-random code used to spread the signal. But more importantly, the receiver must be able to perform a complicated electronic cross-correlation at a pace in keeping with the rate of the code. The use of high speed electronic devices is not necessarily a practical solution due to the high power consumption and complexity associated with such devices.
The present invention discloses an optical solution to the above-identified problem. Instead of using a narrow-band electromagnetic signal, a wideband optical signal is used. This wideband optical signal is then spread by a pseudo-random code. A time delay is added as a means of modulating an information signal onto the spread-spectrum optical signal. The process gain of the present invention is determined by the wideband optical signal rather than the rate of the pseudo-random code.
The present invention discloses a time-hopped ultra wideband optical spread-spectrum communications system. The present invention exploits the jitter-free operation and high repetition rate which is possible with photoconductive switching. The present invention employs a oscillating laser to trigger a fast photoconductive switch, a photoconductive switch to convert the wideband optical signal into an electromagnetic signal which can be broadcast, and an antenna for broadcasting the electromagnetic signal.
The present invention uses optical devices in place of some electronic devices in order to achieve a higher data rate, higher isolation, and higher immunity to electromagnetic interference.
An article entitled "An Impulse Radio Communications System," authored by Paul Withington, II and Larry W. Fullerton, published in a book entitled Ultra-Wideband Short-Pulse Electromagnetics, Plenum Press, 1993, discloses an impulse radio that uses pulse position modulation, where the interval between pulses is based on an information component and a pseudo-random code component. The radio disclosed in this article is based in electronics and, therefore, cannot achieve the high data rates as the present invention which is a spread-spectrum system based in optics. U.S. Pat. No. 4,441,186, entitled ELECTRONICALLY SWITCHABLE MULTIWAVELENGTH LASER SYSTEM, discloses the use of a Pockels' Cell to switch between two wavelengths of a laser. The present invention discloses the use of a Pockels' Cell to select, under the control of a pseudo-random code, various pulses from a pulse train of an oscillating laser. The present invention then adds an information signal to the output of the Pockels' cell. The additive signal is then transmitted to a recipient who knows the pseudo-random code. U.S. Pat. No. 4,441,186 does not disclose such a spread-spectrum communications system.
U.S. Pat. No. 5,264,960, entitled OPTICAL WAVELENGTH SHIFTER, discloses a method of impressing a signal onto an optical signal at one wavelength and then converting this optical signal to a optical signal of a different wavelength. The present invention spreads an intended signal over one time-hopped wavelength of an optical signal. U.S. Pat. No. 5,264,960 does not disclose such a spread-spectrum communications system.
U.S. Pat. No. 4,380,391, entitled SHORT PULSE CO2 LASER FOR RANGING AND TARGET IDENTIFICATION, discloses the use of a Pockels' Cell to chop a laser beam into a train of nanosecond pulses which are then transmitted. The reflections from these pulses are categorized in order to identify the object off of which the transmitted signal was reflected. The present invention discloses the use of a Pockels' Cell to select, under the control of a pseudo-random code, various pulses from a pulse train of an oscillating laser. The present invention then adds an information signal to the output of the Pockels' Cell. The additive signal is then transmitted to a recipient who knows the pseudo-random code. U.S. Pat. No. 4,380,391 does not disclose such a spread-spectrum communications system.
U.S. Pat. No. 5,157,542, entitled OPTICAL FM MODULATION SYSTEM, discloses an optical modulation system. The laser beam is modulated by a signal that has been processed by an integrator. The result is that the laser beam is frequency modulated by the signal impressed upon the integrator. The present invention discloses the use of a Pockels' Cell to select, under the control of a pseudo-random code, various pulses from a pulse train of an oscillating laser. The present invention then adds an information signal to the output of the Pockels' cell. The additive signal is then transmitted to a recipient who knows the pseudo-random code. U.S. Pat. No. 5,157,542 does not disclose such a spread-spectrum communications system.